Android measure过程分析
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时间：2015年06月11日
作为一名Android开发人员，我们都知道一个View从无到有，会经历3个阶段：
1. measure/测量阶段，也就是确定某个view大小的过程；
2. layout/布局阶段，也就是确定其左上右下坐标的过程；
3. draw/绘制阶段，也就是按照前面2步计算的结果，将view绘制在屏幕相应的位置上；
今天，我带领大家来看看View系统的measure过程。到现在相信大部分人都知道measure是从ViewRootImpl.measureHierarchy
方法开始的，但归根结底是从performTraversals开始的。
为了从一开始就清楚onMeasure(int widthMeasureSpec, int heightMeasureSpec)的这2个参数从哪来的，虽然我们都知道
这2个参数表示parent施加给我们的约束，但可能大部分人不明白程序run起来的时候这些值都是从哪里来的。为了弄清楚这个问题，
我们还得从上面ViewRootImpl的measureHierarchy说起，来看其源码：
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp, final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) { int childWidthMeasureSpec; int childHeightMeasureSpec; boolean windowSizeMayChange = false; if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG, "Measuring " + host + " in display " + desiredWindowWidth + "x" + desiredWindowHeight + "..."); boolean goodMeasure = false; if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) { // 在WRAP_CONTENT的情况下，先从一个prefered值开始measure // On large screens, we don‘t want to allow dialogs to just // stretch to fill the entire width of the screen to display // one line of text. First try doing the layout at a smaller // size to see if it will fit. final DisplayMetrics packageMetrics = res.getDisplayMetrics(); res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true); int baseSize = 0; if (mTmpValue.type == TypedValue.TYPE_DIMENSION) { baseSize = (int)mTmpValue.getDimension(packageMetrics); } if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize); if (baseSize != 0 && desiredWindowWidth > baseSize) { // 如果baseSize真小的话，用baseSize先measure一遍试试 childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured (" + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")"); if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) { goodMeasure = true; // measure的结果合适 } else { // Didn‘t fit in that size... try expanding a bit. baseSize = (baseSize+desiredWindowWidth)/2; // 加大baseSize重新执行上述过程 if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize=" + baseSize); childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured (" + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")"); if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) { if (DEBUG_DIALOG) Log.v(TAG, "Good!"); goodMeasure = true; } } } } if (!goodMeasure) { // 如果用baseSize measure的结果不合适，则老老实实用提供的参数重新measure一遍 childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) { windowSizeMayChange = true; } } if (DBG) { System.out.println("======================================"); System.out.println("performTraversals -- after measure"); host.debug(); } return windowSizeMayChange; } /** * Figures out the measure spec for the root view in a window based on it‘s * layout params. * * @param windowSize * The available width or height of the window * * @param rootDimension * The layout params for one dimension (width or height) of the * window. * * @return The measure spec to use to measure the root view. */ private static int getRootMeasureSpec(int windowSize, int rootDimension) { // 通过具体的windowSize和提供的spec int measureSpec; // 构造一个合适的Root MeasureSpec switch (rootDimension) { // 这里的windowSize就是设备的宽、高，rootDimension就是xml文件里指定的layoutparam case ViewGroup.LayoutParams.MATCH_PARENT: // Window can‘t resize. Force root view to be windowSize. 设置root view就是window这么大 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. 设置root view最多是window这么大 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. 某一个具体的大小 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try {
// 调用root view的measure方法，从此进入到view层次结构，顺便也把MeasureSpec带了进去。。。 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } }
从之前的文章中，我们知道DecorView实际上是继承至FrameLayout，由于它和ViewGroup都没有重载这个方法，实际上也没法重载，
因为这个方法是View的一个final方法，代码如下：
/** * <p> * This is called to find out how big a view should be. The parent * supplies constraint information in the width and height parameters. * </p> * * <p> * The actual measurement work of a view is performed in * {@link #onMeasure(int, int)}, called by this method. Therefore, only * {@link #onMeasure(int, int)} can and must be overridden by subclasses. * </p> * * * @param widthMeasureSpec Horizontal space requirements as imposed by the * parent * @param heightMeasureSpec Vertical space requirements as imposed by the * parent * * @see #onMeasure(int, int) */ public final void measure(int widthMeasureSpec, int heightMeasureSpec) { boolean optical = isLayoutModeOptical(this); if (optical != isLayoutModeOptical(mParent)) { Insets insets = getOpticalInsets(); int oWidth = insets.left + insets.right; int oHeight = insets.top + insets.bottom; widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth); heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight); } // Suppress sign extension for the low bytes long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL; if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2); if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT || widthMeasureSpec != mOldWidthMeasureSpec || heightMeasureSpec != mOldHeightMeasureSpec) { // first clears the measured dimension flag mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET; resolveRtlPropertiesIfNeeded(); int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 : mMeasureCache.indexOfKey(key); if (cacheIndex < 0 || sIgnoreMeasureCache) { // measure ourselves, this should set the measured dimension flag back onMeasure(widthMeasureSpec, heightMeasureSpec); // 注意这个调用，这个方法是本文的重点 mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } else { long value = mMeasureCache.valueAt(cacheIndex); // Casting a long to int drops the high 32 bits, no mask needed setMeasuredDimension((int) (value >> 32), (int) value); mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } // flag not set, setMeasuredDimension() was not invoked, we raise // an exception to warn the developer if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) { throw new IllegalStateException("onMeasure() did not set the" + " measured dimension by calling" + " setMeasuredDimension()"); } mPrivateFlags |= PFLAG_LAYOUT_REQUIRED; } mOldWidthMeasureSpec = widthMeasureSpec; mOldHeightMeasureSpec = heightMeasureSpec; mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 | (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension }
正如方法doc说的那样，真正的measure过程是发生在onMeasure方法中的，所以你可以也应该override这个方法，我们紧接着看看View中
的默认实现，代码如下：
/** * <p> * Measure the view and its content to determine the measured width and the * measured height. This method is invoked by {@link #measure(int, int)} and * should be overriden by subclasses to provide accurate and efficient * measurement of their contents. * </p> * * <p> * <strong>CONTRACT:</strong> When overriding this method, you * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the * measured width and height of this view. Failure to do so will trigger an * <code>IllegalStateException</code>, thrown by * {@link #measure(int, int)}. Calling the superclass‘ * {@link #onMeasure(int, int)} is a valid use. * </p> * * <p> * The base class implementation of measure defaults to the background size, * unless a larger size is allowed by the MeasureSpec. Subclasses should * override {@link #onMeasure(int, int)} to provide better measurements of * their content. * </p> * * <p> * If this method is overridden, it is the subclass‘s responsibility to make * sure the measured height and width are at least the view‘s minimum height * and width ({@link #getSuggestedMinimumHeight()} and * {@link #getSuggestedMinimumWidth()}). * </p> * * @param widthMeasureSpec horizontal space requirements as imposed by the parent. * The requirements are encoded with * {@link android.view.View.MeasureSpec}. * @param heightMeasureSpec vertical space requirements as imposed by the parent. * The requirements are encoded with * {@link android.view.View.MeasureSpec}. * * @see #getMeasuredWidth() * @see #getMeasuredHeight() * @see #setMeasuredDimension(int, int) * @see #getSuggestedMinimumHeight() * @see #getSuggestedMinimumWidth() * @see android.view.View.MeasureSpec#getMode(int) * @see android.view.View.MeasureSpec#getSize(int) */ protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } /** * <p>This method must be called by {@link #onMeasure(int, int)} to store the * measured width and measured height. Failing to do so will trigger an * exception at measurement time.</p> * * @param measuredWidth The measured width of this view. May be a complex * bit mask as defined by {@link #MEASURED_SIZE_MASK} and * {@link #MEASURED_STATE_TOO_SMALL}. * @param measuredHeight The measured height of this view. May be a complex * bit mask as defined by {@link #MEASURED_SIZE_MASK} and * {@link #MEASURED_STATE_TOO_SMALL}. */ protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) { boolean optical = isLayoutModeOptical(this); if (optical != isLayoutModeOptical(mParent)) { Insets insets = getOpticalInsets(); int opticalWidth = insets.left + insets.right; int opticalHeight = insets.top + insets.bottom; measuredWidth += optical ? opticalWidth : -opticalWidth; measuredHeight += optical ? opticalHeight : -opticalHeight; } mMeasuredWidth = measuredWidth; // 赋值 mMeasuredHeight = measuredHeight; mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; // 设置标志位 } /** * Returns the suggested minimum height that the view should use. This * returns the maximum of the view‘s minimum height * and the background‘s minimum height * ({@link android.graphics.drawable.Drawable#getMinimumHeight()}). * <p> * When being used in {@link #onMeasure(int, int)}, the caller should still * ensure the returned height is within the requirements of the parent. * * @return The suggested minimum height of the view. */ protected int getSuggestedMinimumHeight() {// 找到view的最小大小，没background的时候返回mMinHeight，这个你可以在xml中指定 return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight()); // 否则返回mMinHeight和background的最小值里的较大者 } /** * Returns the suggested minimum width that the view should use. This * returns the maximum of the view‘s minimum width) * and the background‘s minimum width * ({@link android.graphics.drawable.Drawable#getMinimumWidth()}). * <p> * When being used in {@link #onMeasure(int, int)}, the caller should still * ensure the returned width is within the requirements of the parent. * * @return The suggested minimum width of the view. */ protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); } /** * Utility to return a default size. Uses the supplied size if the * MeasureSpec imposed no constraints. Will get larger if allowed * by the MeasureSpec. * * @param size Default size for this view * @param measureSpec Constraints imposed by the parent * @return The size this view should be. */ public static int getDefaultSize(int size, int measureSpec) { int result = size; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); switch (specMode) { case MeasureSpec.UNSPECIFIED: // parent imposed的spec没指定，则用自己的值default size result = size; break; case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: // 否则不论是精确指定或是至多，都用spec提供的值 result = specSize; break; } return result; }
我们看到View.onMeasure方法只是提供了一个通用的、一般的实现，子类一般需要重载它，自己提供更加合理、高效的实现，最重要的是
符合你的需求。同时我们也看到ViewGroup并没有提供它自己的实现，但是提供了一些在measure过程中很有用的方法，其特定子类如
FrameLayout、LinearLayout等在measure过程中都需要用到的。
为了更进一步看看这个过程，我们这里以FrameLayout的onMeasure为例分析下，看其源码：
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int count = getChildCount(); // matchparent child标记 final boolean measureMatchParentChildren = MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY || MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY; mMatchParentChildren.clear(); // 清空列表 int maxHeight = 0; int maxWidth = 0; int childState = 0; // 遍历children，初步找出maxHeight和maxWidth，顺便构造mMatchParentChildren列表 for (int i = 0; i < count; i++) { final View child = getChildAt(i);
// 默认情况下，只measure非GONE的child，但你可以设置mMeasureAllChildren来打破这一限制 if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 调用parent，ViewGroup提供的方法。。。 measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
// 我们平时在xml文件中设置的android:layout_xxx其实就是这里的LayoutParams final LayoutParams lp = (LayoutParams) child.getLayoutParams(); maxWidth = Math.max(maxWidth, child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin); maxHeight = Math.max(maxHeight, child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin); childState = combineMeasuredStates(childState, child.getMeasuredState()); if (measureMatchParentChildren) { if (lp.width == LayoutParams.MATCH_PARENT || lp.height == LayoutParams.MATCH_PARENT) { mMatchParentChildren.add(child); // 添加matchparent child } } } } // 进一步调整maxWidth、maxHeight的值，考虑foreground padding、minimum height/width，还有foreground的最小值 // Account for padding too maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground(); maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground(); // Check against our minimum height and width maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight()); maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth()); // Check against our foreground‘s minimum height and width final Drawable drawable = getForeground(); if (drawable != null) { maxHeight = Math.max(maxHeight, drawable.getMinimumHeight()); maxWidth = Math.max(maxWidth, drawable.getMinimumWidth()); } // 设置FrameLayout自身的measuredWidth、measuredHeight setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), resolveSizeAndState(maxHeight, heightMeasureSpec, childState << MEASURED_HEIGHT_STATE_SHIFT)); count = mMatchParentChildren.size(); if (count > 1) { // 如果有matchparent child的话 for (int i = 0; i < count; i++) { final View child = mMatchParentChildren.get(i); final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); int childWidthMeasureSpec; int childHeightMeasureSpec; if (lp.width == LayoutParams.MATCH_PARENT) {
// 如果是MATCH_PARENT的话，由于parent已经measure过了，所以就相当于child指定了确定值，
// 所以用的MeasureSpec.EXACTLY。。。 childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() - getPaddingLeftWithForeground() - getPaddingRightWithForeground() - lp.leftMargin - lp.rightMargin,// 在算child大小的时候要去掉parent的padding，child自己指定的各种margin MeasureSpec.EXACTLY); } else {
// 否则，根据parent的measureSpec，已经用掉的大小，child的layoutparam的信息，创建一个合适的MeasureSpec childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, getPaddingLeftWithForeground() + getPaddingRightWithForeground() + lp.leftMargin + lp.rightMargin, lp.width); } if (lp.height == LayoutParams.MATCH_PARENT) { childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() - getPaddingTopWithForeground() - getPaddingBottomWithForeground() - lp.topMargin - lp.bottomMargin, MeasureSpec.EXACTLY); } else { childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec, getPaddingTopWithForeground() + getPaddingBottomWithForeground() + lp.topMargin + lp.bottomMargin, lp.height); } // 用新的childWidthMeasureSpec、childHeightMeasureSpec再次measure child child.measure(childWidthMeasureSpec, childHeightMeasureSpec); } } }
接下来，我们重点看看涉及到的几个ViewGroup方法，代码如下：
/** * Ask one of the children of this view to measure itself, taking into * account both the MeasureSpec requirements for this view and its padding * and margins. The child must have MarginLayoutParams The heavy lifting is * done in getChildMeasureSpec. * * @param child The child to measure * @param parentWidthMeasureSpec The width requirements for this view * @param widthUsed Extra space that has been used up by the parent * horizontally (possibly by other children of the parent) * @param parentHeightMeasureSpec The height requirements for this view * @param heightUsed Extra space that has been used up by the parent * vertically (possibly by other children of the parent) */ protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin // 考虑上parent的padding和child的margin + widthUsed, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed, lp.height); // 这里如果child是个ViewGroup类型，则实际会递归下去。。。 child.measure(childWidthMeasureSpec, childHeightMeasureSpec); } /** * Does the hard part of measureChildren: figuring out the MeasureSpec to * pass to a particular child. This method figures out the right MeasureSpec * for one dimension (height or width) of one child view. * * The goal is to combine information from our MeasureSpec with the * LayoutParams of the child to get the best possible results. For example, * if the this view knows its size (because its MeasureSpec has a mode of * EXACTLY), and the child has indicated in its LayoutParams that it wants * to be the same size as the parent, the parent should ask the child to * layout given an exact size. * * @param spec The requirements for this view * @param padding The padding of this view for the current dimension and * margins, if applicable * @param childDimension How big the child wants to be in the current * dimension * @return a MeasureSpec integer for the child */ public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
// 这个方法是协商型的，最终结果既可能直接由spec（parent提供的），也可能由childDimension决定
// 所以我们知道了，一个View的大小不是简单的单方面决定的，而是通过一系列条件协商的结果，
// 有时会尊重parent的spec，有时会坚持自己的dimension要求 int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding); // 可用的大小 int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: // parent说child你应该是个确定的大小 if (childDimension >= 0) { // child正好设置了确定的大小 resultSize = childDimension; // 让child是那个确定的大小 resultMode = MeasureSpec.EXACTLY; // 设置mode为EXACTLY } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; // 其他情况下都是parent spec中的大小，只是mode不同 resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can‘t be // bigger than us. resultSize = size; // 不能超过size resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: // parent说child你应该最大是某个值。。。 if (childDimension >= 0) { // child指定确定值了，则听child的 // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can‘t be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: // parent没对child的大小有啥要求 if (childDimension >= 0) { // child指定了确定的值，听child的 // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = 0; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = 0; resultMode = MeasureSpec.UNSPECIFIED; } break; } return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }
至此我们已经将measure过程的相关代码大致分析了下，进一步的理解还需要大家在开发中慢慢体会、细细研读源码，最后附上一篇
不错的同主题文章：http://www.cnblogs.com/xilinch/archive/2012/10/24/2737178.html。
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